Github user chtyim commented on a diff in the pull request: https://github.com/apache/twill/pull/58#discussion_r132297305 --- Diff: twill-yarn/src/main/java/org/apache/twill/internal/appmaster/ApplicationMasterService.java --- @@ -193,7 +200,91 @@ private EventHandler createEventHandler(TwillSpecification twillSpec) throws Cla Preconditions.checkArgument(EventHandler.class.isAssignableFrom(handlerClass), "Class {} does not implements {}", handlerClass, EventHandler.class.getName()); - return Instances.newInstance((Class<? extends EventHandler>) handlerClass); + final EventHandler delegate = Instances.newInstance((Class<? extends EventHandler>) handlerClass); + if (delegate == null) { + // if no handler is specified, return an EventHandler with no-op + return new EventHandler() {}; + } + // wrap the delegate EventHandler so that all errors will be caught + return new EventHandler() { --- End diff -- You'll need to wrap the `initialize` as well. Also, I'd suggest if `initialize` throw exception, you'll just ignore all calls to other lifecycle methods to avoid excessive logging.
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